Individual variation in the postcranial skeleton of the Early Cretaceous Iguanodon bernissartensis (Dinosauria: Ornithopoda)
Introduction
Intraspecific variability can be defined as those differences occurring among individuals of the same species regardless of type (morphological, behavioral, ecological, genetic, etc.). Two sources of intraspecific variations can be distinguished: ontogenetic variability (age-dependent) and individual variability (age-independent). Individuals of different ages are subject to both ontogenetic and individual variation; thus, individual variation can only be studied in individuals of approximately the same age and development stage. In non-hadrosaurid ornithopods, studies of morphological ontogenetic variation are rare but usually rather detailed (e.g., Galton, 1974, Norman, 1980, Grigorescu and Csiki, 2006, Hübner and Rauhut, 2010, Prieto-Márquez, 2011, Guenther, 2014, Zheng et al., 2014, Verdú et al., 2015). In contrast, studies of individual variation are even scarcer and less detailed (e.g., Galton, 1974, Norman, 1980, Weishampel et al., 2003, Carpenter and Wilson, 2008, Godefroit et al., 2009, McDonald, 2012a, Carpenter and Lamanna, 2015), including some attempts to identify sexual dimorphism (Nopcsa, 1929, Galton, 1974, Weishampel et al., 2003), despite the importance of individual variation in systematic, phylogenetic and ontogenetic studies in dinosaurs.
The main problem in studies of individual variation in non-hadrosaurid ornithopods is the usually low number of comparable individuals discovered of a single species; thus, the results cannot be considered statistically significant. The Early Cretaceous (late Barremian-earliest Aptian) styracosternan Iguanodon bernissartensis Boulenger in Van Beneden 1881 is represented by an exceptional series of subcomplete specimens from a single locality. In 1878, miners discovered a large number of skeletons in a coal mine in Bernissart (southern Belgium; Godefroit et al., 2012); Norman (1986) estimated that at least 37–38 specimens of I. bernissartensis were unearthed from this ‘Iguanodon Sinkhole’. Most of them are approximately the same size and can be considered ‘adults’, while three smaller specimens may represent ‘sub-adults’. All these skeletons appeared to be clustered into four stratigraphically separated, dinosaur bone beds in an 8-m-thick clay layer (Baele et al., 2012). Accounting for both the stratigraphic and taphonomic data in the ‘Iguanodon Sinkhole’, the most likely scenario for the formation of these four bone bed deposits is a periodic mass-death event (perhaps an intoxication) occurring seasonally in an Early Cretaceous paleolake (Baele et al., 2012). Hence, this high number of specimens from Bernissart is a unique opportunity to study the morphological variation in the skeleton of non-hadrosaurid ornithopods. Furthermore, I. bernissartensis fossils have also been found in other areas of Europe, including the United Kingdom, France, Germany, and Spain (Norman, 2012).
As part of his extensive and detailed monograph about I. bernissartensis, Norman (1980) briefly described some variations in its cranial and postcranial skeleton. Here, we report the results of a more detailed study of the individual variation in the postcranial skeleton of the larger, similarly sized I. bernissartensis specimens from Bernissart (Belgium). However, the smaller ‘sub-adult’ specimens were also examined, and any ontogenetic variation is discussed. These observations are evaluated against those of other specimens of this taxon from other regions of Europe. Moreover, the implications of this work for systematic, phylogenetic, and ontogenetic studies in Iguanodontia are explored. Finally, we discuss the importance of individual variation in I. bernissartensis for evaluating the validity of the early Barremian basal styracosternan ‘Delapparentia turolensis’ Ruiz-Omeñaca 2011 from Spain, which is considered a potential nomen dubium by Norman (2015).
AR – Mina Santa María de Ariño site, Ariño, Spain (the material is currently housed at the Museo Aragonés de Paleontología in Teruel, Spain).
CMP – Cantera Más de la Parreta site, Morella, Spain (the material is currently housed at the Museo de la Valltorta in Tirig, Spain, and the Museo Paleontológico de Elche in Elche, Spain [sensu Gasulla, 2015]).
GPI – Geologisches und Paläontologisches Institute, Münster, Germany.
MAP – Museo Aragonés de Paleontología (Fundación Conjunto Paleontológico de Teruel-Dinópolis), Teruel, Spain.
M-MR – Mas de Romeu site, Morella, Spain (the material is currently housed at the Museu Temps de Dinosaures, Morella, Spain).
MNHN – Muséum National d'Histoire Naturelle, Paris, France.
MPT – Museo Provincial de Teruel, Teruel, Spain.
M-TA – Tejería Azuví site (currently Cantera Azuví), Morella, Spain (the material is currently housed at the Museu Temps de Dinosaures, Morella, Spain).
NHMUK – Natural History Museum, London, U.K.
RBINS – Royal Belgian Institute of Natural Sciences, Brussels, Belgium.
SC – San Cristóbal sites, Galve, Teruel, Spain (the material is currently housed at the Museo Aragonés de Paleontología, Teruel, Spain).
USNM – National Museum of Natural History, Washington D.C., U.S.
Section snippets
Materials
The I. bernissartensis specimens studied in this work were unearthed from the lacustrine facies of the Sainte-Barbe Clay Formation (upper Barremian-lowermost Aptian) in the ‘Iguanodon Sinkhole’ in Bernissart (Belgium) (Yans et al., 2012). These specimens were described in detail by Norman (1980), and although they are all deposited in the paleontological collections of the Royal Belgian Institute of Natural Science (RBINS), specimen RBINS R343 is currently on exhibit in the Musée de l'Iguanodon
Variation in the postcranial skeleton of I. bernissartensis
The most significant individual variation found in the postcranial skeleton of I. bernissartensis is summarized in Table 1. In the following sub-sections, the results are detailed and discussed extensively.
Sexual dimorphism in the postcranial skeleton
Norman (1980) did not find any evidence of sexual dimorphism in either the cranial or postcranial skeleton of I. bernissartensis. Galton (2012) proposed that the pentapleural (free ribs in the first co-ossified vertebra) and hexapleural (ribs of the first co-ossified vertebra incorporated into the sacral yoke) conditions of the sacrum in the basal ornithopod Hypsilophodon foxii might be a potential sexually dimorphic character. The incorporation of the first sacral rib into the sacral yoke has
How individual variation can affect iguanodontian systematics: the case of ‘Delapparentia turolensis’ Ruiz-Omeñaca 2011
Lapparent (1960) recognized the presence of I. bernissartensis in the lower Barremian from Galve (Teruel, Spain) based on the partial postcranial skeleton of a large individual that included several vertebrae and ribs and parts of the ilium, pubis and ischium. More recently, Ruiz-Omeñaca (2011) proposed that this material is sufficiently different from that of I. bernissartensis and should be included in a new taxon, ‘Delapparentia turolensis’. An isolated dentary (MPZ 2014/329) and an ischium
Conclusions
Individual variation in the postcranial skeleton of I. bernissartensis is more important than previously stated. The main quantitative and qualitative morphological variations observed among the different ‘adult’ specimens of I. bernissartensis discussed in this work are summarized as follows: significant variation in the height and (occasionally) shape of the axial neural spine; the number of co-ossified sacral vertebrae (unfused posterior sacral or first caudal incorporated as sacro-caudal);
Acknowledgements
This research was funded by the Departamento de Educación, Cultura y Deporte del Gobierno de Aragón, a Brain-be grant (‘Coldcase’) from Belspo, the Departamento de Innovación, Investigación y Universidad del Gobierno de Aragón and the Fondo Social Europeo (Grupo de Investigación Consolidado FOCONTUR, E62), the Instituto Aragonés de Fomento, the Spanish Ministerio de Economía y Competitividad (CGL2013-41295-P Project DINOTUR), the Ministerio de Educación, Cultura y Deporte (FPU2010 AP-4775,
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